This invention relates to chilling equipment and in particular to improvements in controls for chilling equipment.
A typical large modern office building is commonly air conditioned by circulating chilled water through the building to individual room air handling devices. Large water chilling machines typically have centrifugal type compressors used to chill the water. Such machines are usually coupled with the building electrical system through a conventional electro-magnetic starter. The starter itself may cost from $1,500 to $8,000, and contains numerous heavy power wires and electrical components such as contactors, overload relays, control and current transformers, time delay and control relays. These items have moving parts and open contacts. They develop heat and are subject to deterioration, the pitting and burning of the contact points being a frequent problem. If the starter door can be opened, debris and dust accumulation seem to be inevitable. Periodic maintenance should be maintained also to guard against the effects of vibration and insulation aging, but unfortunately, maintenance is usually advocated more than it is practiced.
At the job site the electrical contractor must interconnect the wiring between the chiller and the standard type starter furnished by the mechanical contractor, sometimes over distances of 50 feet and more. Frequently, there is misunderstanding between the electrical and mechanical contractors or with the specifying engineer. Various makes of chillers and starters may be used together and may require mounting of special small controls and relays in the starter. This information is not always known in advance, leading to unpleasant surprises.
In spite of difficulties with the electro-mechanical starter, it has heretofore provided the standard and only means of starting and stopping large motors of the type used in water chillers.
The present invention is directed toward a new solid state starter especially useful with large centrifugal chillers. The invention overcomes many of the disadvantages of the conventional electro-mechanical starter.
One feature of the new solid state starter, according to the invention, is that it is mounted on the chiller compressor. All control and power wires are connected at the factory, and the complete assembly can be operated under load with a chiller before it leaves the factory. The floor space saved by the invention is an important consideration in crowded equipment rooms.
Another feature of the new starter is the use of silicon controlled rectifiers, or SCR's, for controlling current to the compressor motor. Each phase utilizes two SCR's: one for the upper half of the electrical current sine wave; the other, for the lower half of the sine wave. Six are used for each three phase machine. The SCR's turn the current off and on -- silently and with no moving parts. The gate trigger signal pulses the SCR at the desired electrical phase angle. This current flow is extinguished on each half cycle as the sine wave crosses zero. The control circuits not only turn the chiller motor off and on but can control the amount of current. Three rectangular silicon controlled rectifier clusters, with two SCR's each, are mounted in a sealed starter box attached to the motor terminal box. Liquid refrigerant flows through the starter box to hold the temperature of the SCR's under 150.degree. F. This arrangement thus provides silent on-off switching of current by a completely sealed mechanism with no moving parts and no exposure to debris and dust or inexpert tampering. In contrast, a conventional electro-mechanical starter has many moving parts, oil dash pot overload relays, springs, and contacts. It clatters shut with highly visible arcing and noise.
A further feature of the invention relates to the electronic control modules, all solid state, which are included with the new starter. One of these is a trigger module which triggers the SCR's at controlled firing angles in the line voltage cycle to regulate the current flow to the large chiller motor. It receives its control signals from a current module. The current module senses actual motor current and regulates delivery of same to the motor during the starting cycle. The circuit provides rigid programmed control of starting current with a smooth steady change in SCR firing angle during motor acceleration. The module also provides current overload protection. A third control module is the voltage safety module. It contains three safety circuits which protect the motor against electrical emergencies, specifically against three phase voltage unbalance, undervoltage and phase reversal.
Each control module contains a circuit board assembly inserted into a suitable sheet metal enclosure. The enclosure is filled with a potting compound and is sealed closed. All circuits are thereby protected against contamination from dust, moisture, vibration, and tampering. External terminals provide for connection of wires to the module circuit.
The three modules just described mount in a motor control panel which is disposed in front of the starter box. The starter box and the motor control panel along with a terminal box and a control connection box, are mounted together on the compressor. They are designed for convenient access to all internal components and to minimize wiring complexity.
One outstanding benefit of the new starter is its outstanding reliability. The reliability is provided in part by factory mounting the starter on the chiller thus eliminating mistakes due to field wiring. U.S. Underwriters Laboratory approval of both starter and chiller and the combination is a first, an indication of unusual safety and reliability. Reliability also accrues because all controls are hermetically sealed from contamination and inexpert tampering. Sold state construction eliminates moving parts.
Another major benefit is lower cost for a completely installed system. While the solid state starter components may be more expensive than the corresponding electro-mechanical components, the completely installed system will be less expensive. This is because virtually all field wiring is eliminated. Another cost savings is reduction in floor space. Another cost benefit is that start-up costs are reduced. Since the combination chiller and starter can be factory tested together, they can be delivered as a complete package ready for connection to the building electrical supply. Start-up procedures are greatly simplified and shortened. Back charges are reduced. With electro-mechanical starters installed by electrical contracting specialists, there arise frequent disputes over field mounting of control components, responsibility for cost of changes, and additions to the control wiring. All these problems are eliminated by the present invention.
Another major benefit is that the starter provides many extra performance, safety and reliability features, some of which are available in electro-mechanical starters but only at substantially extra cost. The solid state control provides much better starting current control than across-the-line, autotransformer, or star-delta closed transition starters.
The solid state current module provides a smooth start with current never exceeding about 170% of full load under normal conditions. This permits substantially lower electrical operating costs where electric utilities charge higher rates for higher starting current surges. The design of the current control package of the solid state control permits the motor to come up to full speed even if voltage at the starter falls 35% below normal. An average star-delta starter will not bring the motor up to speed if the voltage drops more than 10% below normal. By eliminating high starting current spikes, electrical line transients in electrical distribution systems are attenuated.
Another extra performance feature is factory-set overloads. The current module is set, with a limited field adjustment, to interrupt the voltage applied to the motor if a current in excess of 200% rated load current is encountered during the starting period. After starting, a fault will also be indicated on the starter instrument panel if the current exceeds 120% of rated load current use since current should remain under 105%. This overload control is not temperature sensitive. Unlike mechanical starters, it is factory set and tested.
Longer motor and starter life through excellent protection is another feature of the solid state starter. In the voltage control module the following faults are immediately recognized and the motor is protected against them: voltage phase unbalance exceeding 2% for more than one second; primary signal phase fault, both in power source transformer and in primary distribution supply; secondary single phase fault; SCR short or open; control circuit fault; motor short, open, or ground; voltage more than 15% below rated; accidental phase reversal.
Additional features, benefits and advantages will be seen in the following description and claims which are to be considered in conjunction with the accompany drawings illustrating a preferred embodiment of the invention according to the best mode presently contemplated in carrying out the several aspects of the invention.